An optical transmitter is provided at the front end of an optical communication system. The optical transmitter converts electrical signals to optical signals and outputs the result.
Typically, it is preferable for the strength of the optical signal output by the optical transmitter to always be constant regardless of the passage of time or the temperature of the environment, in order to ensure stability in the optical system. In order to automatically control the strength of the optical signal to a constant value, an APC (Automatic Power Control) is widely used in general.
An optical transmitter utilizing an APC is disclosed in Patent Literature 1.
In the optical transmitter of Patent Literature 1, the strength of the optical signal produced by a light-emitting element is detected by an optical detector. The optical detector generates a current signal corresponding to the strength of the optical signal, and this current signal flows to a variable resistor. The variable resistor converts the current signal to a voltage signal and supplies this voltage signal to a controller. The controller compares the voltage signal output by the variable resistor with a reference voltage and, based on this comparison result, sets the resistance value of the variable resistor, a bias current driving the light-emitting element and a current value of a modulated current. That is to say, the optical transmitter of Patent Literature 1 accomplishes a feedback-type APC to cause stability in the strength of the optical signal generated by the light-emitting element.
With the optical transmitter of Patent Literature 1, the conversion gain when converting the current signal to a voltage signal is set by the resistance value of the variable resistor. Furthermore, the resistance value of the variable resistor is controlled based on the temperature inside the optical transmitter, and the conversion gain when converting the current signal to the voltage signal is regulated.